JP5242416B2 - Method for preparing a metal structure suitable for semi-molten metal processing - Google Patents

Description

  This application claims the benefit of US Provisional Patent Application No. 60 / 764,348, filed February 2, 2006, the entire disclosure of which is incorporated herein by reference.

  The present invention relates to a method for preparing non-dendritic metal slurries for semi-molten casting and forming. In particular, the invention involves flowing gas bubbles through at least one solid medium inserted or contained in a liquid metal or alloy, cooling the liquid metal or alloy below its melting temperature, and stirring the liquid metal or alloy with the gas bubbles. A solid fraction is formed.

  A suitable metal structure (structure) for semi-molten metal (SSM) processing is a doctoral dissertation by Dr. DB Spencer titled “Rheology of Lead / Tin Liquid / Solid Mixtures” in the early 1970s. MC Flemings) (June 1971). They mechanically stirred the solidified alloy in the solid / liquid temperature range and found that the solid phase was not dendritic but spherical particles. The non-dendritic character of the solid phase imparted intrinsic fluidity to these metal “slurries”. Metal slurries containing up to 50% solid phase flow uniformly in the order of “effective viscosity” that is larger than liquid alloys. If the metal slurry is formed into parts, the higher viscosity can reduce turbulence in mold filling and produce high quality parts by minimizing air entrapment and encapsulation.

  Since the invention of SSM processes, the industry has become increasingly aware of the potential of these processes. Today, SSM research is carried out globally in class and industry. The spread of SSM applications in the industry, however, is still in its early stages. The application of aluminum alloys to automobiles has been an important focus of industrial interest in SSM. Fuel efficiency and environmental concerns have become the driving force for lighter vehicles. This continued to increase the use of aluminum castings in vehicles and increased demand for treatments such as SSM that resulted in high quality aluminum alloys. Some of the automotive parts considered for aluminum SSM are SPMidson and SPMidson in a paper titled “Semi-molten casting of aluminum alloys (current report)” (Hyundai Casting, February 1997, pages 41-43). Suspension components, air conditioner compressors, and master brake cylinders described by K. Brissing. Commercial vehicle manufacturers used a fluid casting process to produce cylinder blocks for the next line of turbo diesel engines. See “Development of high-strength aluminum cylinder blocks for diesel engines using a new production process” (SAE International, publication 2004-01-1447), M. Yamazaki et al.

  Two semi-molten metal processing routes, namely “thixocasting” and “fluid casting” are industrially feasible. Thixo casting is a process obtained by reheating billets (steel pieces) in which non-dendritic structures (structures) are sufficiently solidified in the solid / liquid temperature range and forming them into parts. Fluid casting is a process in which a slurry having a non-dendritic structure is made from a liquid alloy and formed into parts.

  Over the past 30 years, the SSM processing route used in the industry has been (at the World Foundry Conference held in Kynogju, October 20-24, 2002) "Highly viscous liquid and semi-molten metal casting: processing and It was thixocasting submitted by MCFlemings and WLJohnson et al. An electromagnetically stirred billet is a paper titled “Details of Semi-molten Alloys by New Electromagnetic Fluid Casting” (Process, Metallurgical Conference B, (23b), April 1992, pp. 189-206). Manufactured on continuous casters as described by Vives. Thixocaster purchases these billets and reheats them in the solid / liquid temperature range to form them into parts. Even when high quality aluminum alloys were obtained, problems such as operational costs and process control prevented widespread application of thixocasting. In 2000, thixoprocessing was published in North America, Europe in a paper titled “Near net forming by semi-molten metal processing” (Materials and Design, (21), 2000, 387-394) and P. Kapranos et al. It was estimated to be only 1% of 2.5 million tons of Japanese aluminum casting.

  Thus, a recent trend in semi-molten metal processing is to advance the fluid casting route. Fluid casting has an immediate cost advantage over thixocasting because liquid alloys are formed into non-dendritic metal slurries at the manufacturing site and scrap metal can be recycled in-house. Today, several processes are available for producing non-dendritic structures from liquid alloys. The first method used is to mechanically stir the metal in the solid / liquid phase temperature range. Several patents, including US Pat. Nos. 5,555,926, 5,887,640, and 5,983,978, describe devices designed to produce metal slurries by mechanical stirring. doing. See also "Semi-Melting Process of Industrial Alloys by Biaxial Flow Molding" (Material Science and Engineering A, (299A), 2001, 210-217) S.Ji, Z.Fan, M.J.Bevis. However, the lack of a strong stirring material that can withstand exposure to molten aluminum for extended periods of time has limited the use of this mechanical stirring method in the industry.

  By this time, the challenge to fluid casting has been limited knowledge on how to efficiently process liquid alloys to produce non-dendritic metal slurries. It has been found that non-dendritic structures can be easily formed by controlling the conditions during the initial stage of solidification (formation of only the first few percent of the solid phase). Through a combination of local rapid cooling and vigorous stirring of the melt, the temperature drops from just above the liquidus to below, and RAMartinez says “a new technology for the formation of semi-molten structures” (June 2001). ) And his doctoral dissertation entitled “Molding and Processing of Flowing Microstructures” (MCFlemings instructor at Massachusetts Institute of Technology) Obtained in seconds.

  In US Pat. No. 6,645,322, Flemings patented a method for effectively fluid casting alloys with spherical particles by immersing a cooled rotating rod in a melt maintained above the liquidus. The intrusion of the rotating bar simultaneously creates a region of high local cooling, provides active convection, and lowers the bulk melt temperature below the liquidus. This process can produce slurry from liquid metal with large overheating fluctuations, making it a robust and effective method for producing aluminum alloy slurry. However, since the method requires a moving solid medium, several problems are anticipated. First of all, it is not easy to continuously apply the water cooling system to the rod during the rotation of the rod. Sensors such as temperature sensors are not easy to install in the rod for temperature measurement. Furthermore, vortices can be formed during the rotation of the rod. The formation of vortices increases the metal oxidation. In order to avoid these problems, it is desirable to have a method that does not require rotation of the solid medium.

  The present invention shows that when a combination of local cooling and active convection is applied to a lysate maintained just above its liquidus temperature, a non-dendritic structure can be formed in a few seconds after initiation of solidification. And uses the principle presented by Flemings. In the present invention, it has been discovered that a non-dendritic, semi-molten metal slurry can be obtained by flowing bubbles through a solid material through a molten metal alloy maintained at a temperature equal to or higher than the liquidus temperature. In the present invention, gas bubbles provide active convection and at the same time provide some local cooling. Local cooling is achieved through the use of solid objects. In contrast to the prior invention, the present invention uses gas bubbles as a medium to stir rather than solid objects such as impellers or cylindrical rods as in the prior art.

  In one aspect, the present invention describes a method for preparing a non-dendritic semi-molten metal slurry by introducing gas bubbles into a molten metal alloy maintained at a temperature above the liquidus temperature through a solid. In the present invention, solids do not rotate, so several advantages are expected. If non-rotating parts are used, the cooling system and sensor system can be applied with a simple design. Since no vortices are formed in the molten metal alloy, enhanced oxidation of the metal by the vortices is avoided. When porous solids are used to provide gas bubbles, the gas bubble flow from the pores of the solid medium acts as a protective layer between the molten metal alloy and the medium surface, so that the gap between the molten metal alloy and the attacked medium Wetting and reaction are avoided. Furthermore, the gas bubble flow within the molten metal alloy helps remove impurities from the slag, dissolved gas, and molten metal alloy, and these widely used processes are known as degassing or deslagging processes.

  The foregoing and further features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawings, in which like reference numerals designate like elements.

FIG. 1 shows an apparatus for preparing a non-dendritic semi-molten slurry according to an embodiment of the present invention. The apparatus includes a holding container for receiving and holding a molten metal alloy, a lance (hollow cylindrical tube) for providing inert gas bubbles, and a solid object (object) for providing more localized cooling. Yes. In one process, the lance (gas blowing tube) is attacked by a molten metal alloy maintained at a temperature above the liquidus temperature. The inert gas is flowed through a lance that generates gas generating bubbles while the solid is lowered into the molten metal alloy. The gas is selected from the group consisting of nitrogen, argon, carbon dioxide, and mixtures thereof. The metal alloy is selected from the group consisting of an aluminum alloy, a magnesium alloy, a copper alloy, an iron alloy , a zinc alloy, a nickel alloy, and a titanium alloy.

As the temperature of the metal alloy falls below the liquidus, a non-dendritic semi-molten metal alloy slurry is formed. When the metal alloy slurry has about 1% to about 50% solids by weight, the solids are removed and the gas flow is stopped. Alternatively, the gas flow may be stopped when the metal alloy slurry has a solid component of about 1% to about 20% by mass.

  Other variations of the invention are possible, such as pipes, impellers, rods, or containers. Some of these are given in FIGS. 2-4, and FIG. 2 shows another embodiment. In this process, the inert gas flows through a solid nozzle. In another embodiment, in FIG. 3, the inert gas flows through the porous solid. In this case, fine and uniform bubbles can be obtained. FIG. 4 shows another embodiment. In one process, the inert gas flows through a porous solid disposed on the wall. Micro gas bubbles can be introduced on all wall surfaces, bottom surfaces or only part of these surfaces. The solid can be formed from graphite, ceramic, metal, or a composite of these materials.

  The molten metal or alloy is cooled by the gas bubble stream and by contact with the solid material. Since one or more of the above solid objects can be placed simultaneously, the molten metal or alloy can be cooled by contacting the plurality of solid objects. In addition, the solid is cooled by flowing air, water, or any coolant through the object.

  In addition to the cooling and stirring functions, the gas bubbles prevent solids from reacting with the metal or alloy and remove impurities from the slag, dissolved gas, or metal or alloy.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification or practice of the invention disclosed herein.
Example 1
A357 alloy processed using a porous graphite diffuser

  The following is a detailed description of a method for preparing an A357 non-dendritic semi-molten slurry, with reference to FIG.

About 520 grams of aluminum alloy A357 (Al-7.0 wt% Si-0.5 wt% Mg) was melted in a stainless steel crucible covered with boron nitride placed in an electric furnace. A porous graphite rod (2.54 cm OD, 1.5 cm OD, length 15.24 cm, 2.6 g / cm ³ ) was processed to form a diffuser. The wall of the graphite diffuser was approximately 10% open porosity. The graphite diffuser was connected to an argon gas cylinder attached to the gas flow meter to give fine gas bubbles to the melt. The alloy was melted and heated at 630 ° C. The melt was slowly cooled slowly to 625 ° C. at a cooling rate of about 1 ° C./min, and the diffuser was quickly immersed by introducing fine argon gas bubbles at a volumetric flow rate of about 2 liters / min. The foaming process is performed until a solid phase of about 10% of the melt is achieved, after which the diffuser is quickly removed allowing the melt to cool slowly. When the melting temperature reached 580 ° C. (about 45% solid fraction), the metal flakes in the crucible were removed and quenched in water.

Samples were polished and examined with an optical microscope. For comparison, FIG. 5 is provided to show a typical photomicrograph of the untreated dendritic microstructure (tissue). The micrograph shows a coarse particle structure of 400 μm or more. In contrast, FIG. 6 shows a typical photomicrograph of a non-dendritic semi-molten structure processed by this method. In this method, the particle structure is finer below 200 μm.
Example 2
A357 alloy processed using lance

  The following is a detailed description of the method for preparing a non-dendritic semi-molten slurry of A357 alloy with reference to FIG.

  About 520 grams of aluminum alloy A357 (Al-7.0 wt% Si-0.5 wt%) was melted in a stainless steel crucible covered with boron nitride placed in an electric furnace. A stainless steel tube (0.4 cm ID, 0.6 cm OD) was processed to form a lance as shown in FIG. The end of the tube was mechanically closed and a small nozzle was machined. The lance was coated with boron nitride and connected to an argon gas cylinder attached to the gas flow meter to provide fine gas bubbles in the melt. The lance was immersed in the melt while the alloy was heated to 630 ° C. The melt was slowly cooled to 625 ° C. at a cooling rate of about 1 ° C./min, and the solid copper cooling body coated with graphite was rapidly immersed with fine argon bubbles simultaneously introduced through the lance (FIG. 1). The volume flow rate was about 1.5 liters / minute. Foaming was performed until a solid phase of about 5% of the melt was obtained, after which the solid copper cooling body was quickly removed and gas flow was stopped. The melt was slowly cooled. When the metal temperature reached 580 ° C. (about 45% solid fraction), the metal flakes in the crucible were removed and quenched in water. The sample was then polished and examined with an optical microscope. FIG. 7 shows a typical micrograph of the microstructure (structure).

  The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. However, the present invention for the purpose of protection is not limited to the specific embodiments disclosed. The embodiments described herein are illustrative rather than limiting. Variations and changes may be made by other means and equivalents employed without departing from the spirit of the invention. Accordingly, it is manifestly intended to embrace all such modifications and variations and equivalents that fall within the spirit and scope of the invention as defined by the appended claims.

FIG. 1 shows a first embodiment of an apparatus for preparing a non-dendritic semi-molten metal slurry according to the present invention. FIG. 2 shows a second embodiment of an apparatus for preparing a non-dendritic semi-molten metal slurry according to the present invention. FIG. 3 shows a third embodiment of an apparatus for preparing a non-dendritic semi-molten metal slurry according to the present invention. FIG. 4 shows a fourth embodiment of an apparatus for preparing a non-dendritic semi-molten metal slurry according to the present invention. FIG. 5 shows a typical photomicrograph of a dendritic microstructure provided without applying the present invention. FIG. 6 shows a typical photomicrograph of a non-dendritic semi-molten structure provided in accordance with the present invention. FIG. 7 shows a typical photomicrograph of another microstructure provided by the present invention.

Explanation of symbols

10 Press 20 Upper core rod

Claims (21)

In a method of using gas bubbles to form a semi-molten metal or alloy having a non-dendritic granular structure,
A first step of heating the metal or alloy above the melting temperature to provide a liquid metal or liquid alloy;
A gas bubble is caused to flow through at least one solid medium inserted or contained in the liquid metal or liquid alloy, and the liquid metal or liquid alloy is cooled to a temperature equal to or lower than the melting temperature while stirring the liquid metal or liquid alloy. And a second step of forming a solid fraction;
When the weight of the solid fraction reaches a range of 0.01 to 0.5 of the weight of the metal or alloy, the gas bubbles are stopped from flowing, and a semi-molten metal or semi-molten alloy having a non-dendritic granular structure is obtained. A third step of providing;
A method comprising the steps of:   The method according to claim 1, wherein when the weight of the solid fraction reaches a range of 0.01 to 0.2 of the weight of the metal or alloy, the gas bubbles are stopped.   The method of claim 1, wherein the cooling is further accomplished by contacting the liquid metal or alloy with the solid medium.   The method of claim 1, wherein the step of flowing gas bubbles provides a cooling rate of at least 1 degree Celsius per minute.   4. The method of claim 3, wherein the cooling is further achieved by contacting the liquid metal or liquid alloy with at least one other solid medium different from the solid medium.   4. A method according to claim 3, wherein the solid medium is cooled by means of flowing air or coolant through it.   The method of claim 1, wherein the solid medium is prevented from reacting with the metal or alloy by the gas bubble stream. The method of claim 1, wherein the solid medium does not move or rotate .   The method of claim 1, wherein the step of flowing the gas bubbles further removes slag, dissolved gas, and impurities from the liquid metal or liquid alloy. In a system for forming a semi-molten metal or alloy having a non-dendritic granular structure using gas bubbles,
Means for heating the metal or alloy above the melting temperature to provide a liquid metal or liquid alloy;
Flowing gas bubbles through at least one solid medium introduced or inserted into the liquid metal or liquid alloy, and stirring the liquid metal or liquid alloy with the gas bubbles, the liquid metal or liquid alloy is brought to the melting temperature or lower. Means for cooling and forming a solid fraction;
When the weight of the solid fraction reaches the range of 0.01-0.50 of the weight of the metal or alloy, the gas bubble flow is stopped, and the semi-molten metal or semi-molten alloy having a non-dendritic granular structure is obtained. Means to provide;
The system characterized by comprising.   The system of claim 10, wherein the metal or alloy is billet-shaped.   The system according to claim 10, wherein the solid medium is made of graphite, ceramic, or a composite thereof.   The system of claim 10, wherein the solid medium supplies the gas bubbles through at least one outlet.   The system of claim 13, wherein the solid medium is a pipe, impeller, rod, or container. The system of claim 13, wherein the solid medium does not move or rotate .   14. The system of claim 13, wherein the solid medium is porous with a gas outlet.   The system of claim 10, further comprising at least one temperature sensor.   The system of claim 17, wherein the temperature sensor is attached to the solid medium.   The system of claim 16, wherein the solid medium is a porous graphite rod with 10% open porosity.   The system of claim 10, wherein the gas is selected from the group consisting of nitrogen, argon, carbon dioxide, and mixtures thereof.   The system of claim 10, wherein the alloy is selected from the group consisting of an aluminum alloy, a magnesium alloy, a copper alloy, an iron alloy, a zinc alloy, a nickel alloy, and a titanium alloy.

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